In freshly isolated rabbit SANC, Protein Phosphatase Inhibition (PPI) by the PP1/2A inhibitor, Calyculin A, (100-500 nM) reduced PP activity by 90%, and increased basal PLB phosphorylation at Thr17 and Ser16 by about 2.5-fold. PPI increased: the rate of spontaneous Ca2+ release of the LCR ensemble (measured via confocal fluo-4 imaging) by nearly four-fold in saponin-permeabilized SANC;the L-type Ca2+ current (ICaL) amplitude by 30% in voltage-clamped, single SANC; and the LCR size in spontaneously firing single, intact SANC. PPI also decreased the LCR period, and this reduction predicted a concurrent 25% reduction in the spontaneous AP cycle length. A numerical model simulation of the effect of PPI on SANC firing rate, incorporating experimental observed changes in ICaL and PLB phosphorylation effects on SR Ca2+ pumping, closely predicted the experimental results. Conclusion: Thus, basal PP activity modulates spontaneous SANC AP firing rate, in part at least, by modulating ICaL, PLB phosphorylation, and SR-generated LCRs.